The present invention relates generally to gas turbine engines, and, more specifically, to turbine nozzles therein.
In a gas turbine engine air is pressurized in a compressor and mixed with fuel in a combustor for generating hot combustion gases. A high pressure turbine (HPT) extracts energy from the hot gases to power the compressor. And, a low pressure turbine (LPT) extracts additional energy from the gases to power an upstream fan in an aircraft turbofan engine application, or to provide external power through a drive shaft for marine and industrial applications.
The HPT includes a first stage turbine nozzle disposed at the outlet of the combustor that first receives therefrom the hot combustion gases which are then directed by the nozzle vanes into a row of first stage turbine rotor blades extending outwardly from the perimeter of a rotor disk. The blades extract energy from the gases to rotate the disk, which in turn rotates the rotor blades of the compressor.
In order to withstand the hot combustion gases, the various components of the HPT are typically formed of superalloys which maintain their strength at elevated temperatures. Furthermore, the nozzle vanes and rotor blades are typically hollow and provided with cooling circuits therein through which is circulated a portion of the air pressurized by the compressor for cooling the vanes and blades during operation.
The prior art includes a myriad of cooling circuits and features specifically configured for the stator vanes of the nozzle and the rotor blades disposed downstream therefrom.
However, any air bled from the compressor for cooling the turbine components is not used during the combustion process and correspondingly reduces the efficiency of the engine. Engine efficiency is directly related to the temperature of the combustion gases generated during operation, with higher combustion gases being used for increasing efficiency of the engine.
Modern gas turbine engines exploit the strength of the superalloy metal vanes and blades and the internal cooling thereof for maximizing engine efficiency, while also obtaining a long useful life of the engine. However, further gains in engine efficiency are limited by the available superalloys and by the amount of cooling air which may be practically bled from the compressor.
Accordingly, current developments in further advancing the efficiency of gas turbine engines include the selective use of ceramic components which can withstand substantially greater temperatures of combustion gases than presently experienced by modern superalloy metals. One type of ceramic material for a gas turbine engine is ceramic matrix composite (CMC) in which silicon carbide fibers are embedded in a silicon carbide matrix for strength and durability.
However, ceramic materials lack ductility and require special mounting to prevent excessive stress therein which could lead to their brittle failure and correspondingly short useful life.
For example, ceramic materials in a gas turbine engine would necessarily be used in conjunction with conventional metal components of the same engine. The ceramic components may be preferentially utilized in the direct flowpath of the hot combustion gases and supported in metal components which do not experience the high heat loads from the combustion gases.
This presents a significant design problem since the ceramic materials have a relatively low coefficient of thermal expansion compared with metal components which expand and contract as temperatures increase and decrease during the various portions of the engine cycle.
In view of the substantial difference in coefficients of thermal expansion between the ceramic material and supporting metal components, substantial thermal stress can be generated in the ceramic material leading to the short life thereof.
Furthermore, the first stage turbine nozzle vanes are also subject to the aerodynamic or pressure loading from the hot combustion gases which must also be carried from the vanes into their supporting components.
Accordingly, it is desired to provide a turbine nozzle with ceramic vanes mounted in metal components which accommodate the different coefficients of thermal expansion therebetween.